1. Field of the Invention
The present invention relates to an image forming apparatus of tandem type in which plural copying portions are connected in series to perform a duplex printing or copying on both surfaces of a sheet of paper including an envelope.
2. Description of Related Art
An image forming apparatus of tandem type in which plural copying portions are connected in series to perform a duplex printing or copying or the like has been put into practical use in recent years. For example, in the image forming apparatus of tandem type in which two copying portions are connected in series, a copying portion at upstream side forms an image on a surface of a sheet of paper; an intermediate transporting portion provided between the copying portion at the upstream side and a copying portion at downstream side reverses a surface of the sheet of paper; and the copying portion at the downstream side forms an image on a back surface of the sheet of paper.
Japanese Patent Application Publication No. H08-234605 has disclosed, as each an image forming apparatus of tandem type in which two copying portions are connected in series, a duplex printing machine in which two electrophotography apparatuses are connected in series, wherein a near roller and a heater are standardized in a first electrophotography apparatus for printing an image on a surface of a sheet of paper and a second electrophotography apparatus for printing an image on a back surface of the sheet of paper.
Such an image forming apparatus of tandem type has been widely used even so a case where an irregular sheet of paper such as an envelope is printed or copied in addition to any regular sheet of paper of A3 size or the like. When printing or copying an image on the envelope, a fixing condition or the like such as temperature control, fixing speed, nipping width by fixing rollers and a nipping pressure thereby is different from that used when printing or copying an image on the regular shoes of paper. Therefore, a special fixing unit for the envelope is used in addition to a standard regular fixing unit. Accordingly, when printing or copying an image on an envelope, the envelope is printed or copied after the regular fixing unit fitted into a fixing unit fitting portion of each copying portion has been exchanged to the fixing unit for the envelope.